It is known in the art to manufacture convoluted foam, for example, polyester, polyether, polyvinyl chloride or urethane foam. Machines for the manufacture of convoluted foams of various profiles, both symmetric and unsymmetric, are commercially available, for example, from Fecken-Kirfel KG located in Aachen, West Germany. In such machines, a sheet of flexible polyurethane foam having a thickness, for example, of from 11/2 to 8 inches can be passed between a pair of rollers having the desired surface configuration or pattern. These rollers squeeze the sheet of foam, and immediately after passing between the rollers and while still in a state of patterned compression the sheet is cut in two by means of a knife to provide two sheets, one side of each of which is convoluted and the other side of each of which has not been changed by passing through the machine.
Flexible foam sheet is convoluted as described because the convoluted foam provides more filtration surface for the same perimeter and because convoluted foam weighs only about one-half as much as unconvoluted foam of the same superficial volume. Foam is impregnated in order to increase its efficiency as a filter and also to increase its rigidity. Thus, air filters are conventionally manufactured with a metal frame to provide support for the filter. The present invention makes possible the manufacture of foam of sufficient rigidity that air filters embodying it are sufficiently rigid that no metal frame is required.